Manufacture of densified spheroidal nitrocellulose of high viscosity at casting temperatures



United States Patent MANUFACTURE OF DENSHFIIED SPHERGIDAL Ni- TRO CiELLUL DSE OF HIGH VHSCOSKTY AT CAST- ENG TEMPERATURES John If. Sapiego, New Brunswick, NA, assignor to Hercules inc, a corporation f Delaware No Drawing. Filed Dec. 114, 1965, fier. No. 513,830 6 Ciairns. (Ci. 2643) This invention relates to the manufacture of tiny particles of densified nitrocellulose for use in casting large propellant grains by the slurry casting process, and more particularly to the manufacture of tiny particles of densified nitrocellulose having improved pot life in slurry casting processes for preparation of large propellant grains. The particles of densified nitrocellulose produced in accordance with this invention are well adapted for use in slurry casting processes which employ elevated mixing and casting temperatures in the range from about 90 F. to about 140 F. because of their improved pot life characteristics.

In my copending application Ser. No. 358,366, now Patent No. 3,236,702, there is described a process for transforming water-moist fibrous nitrocellulose directly into tiny, hard, densified, substantially rounded particles of nitrocellulose wetted with liquid hydrocarbon in place of water. The process described and claimed in copending application Ser. No. 358,366 comprises forming a slurry with agitation of water-moist nitrocellulose fibers substantially free of fiber aggregates in a volatile organic liquid mixture of hydrocarbon diluent and nitrocellulose solvent which is miscible with said diluent in the presence of a water-soluble protective colloid, said organic liquid mixture being only a softening and swelling agent for said nitrocellulose fibers incapable of dissolving said fibers, said solvent being soluble in water to the extent of at least about 2.5% by weight, subjecting the slurry of nitrocellulose fibers to high shear agitation, whereby said fibers become softened and swollen and break down into tiny smooth-surfaced, rounded particles of substantially ellipsoidal and spheroidal shapes, initiating hardening of the comminuted and shaped particles of softened and swollen nitrocellulose by diluting the slurry with hydrocarbon diluent while continuing high shear agitation, and removing substantially all of the nitrocellulose solvent and water by distillation with agitation in the presence of excess hydrocarbon diluent to complete hardening and densification of the nitrocellulose particles.

The above process produces a hydrocarbon-wet product of tiny, hard, densified, substantially ellipsoidal and spheroidal particles of nitrocellulose, a substantial majority of which are between about 40 microns and about 75 microns in size, with an absolute density of at least about 1.3 gms./ cc. and a bulk density of at least about 40 lbs./ cubic foot. When magnified, these particles are seen to have smooth, dense, glazed surfaces, and, generally, to have some degree of porosity in the interior of the particles. These particles have been demonstrated to have desirable particle shape, size and size distribution, density, surface and internal structure, and bulk density characteristics, combined with a satisfactory pot life for use in slurry casting of large propellant grains at ordinary room temperatures.

Pot life is the term employed in the propellant slurry casting art to designate the time period during which the propellant ingredients can be satisfactorily mixed together to form a uniform, pourable slurry which can then be poured into the mold without excessive thickening of the slurry. A test procedure which has been used heretofore for determining whether a fine particle densified nitrocellulose product will produce casting slurries with satisice factory pot life consists of determining the time-viscosity relationship at 20 C. of a slurry containing 30 parts by weight of the densified nitrocellulose particles in 70 parts by weight of triacetin, a solvent-type plasticizer for nitrocellulose, which is commonly employed as a densensitizing plasticizer in the manufacture of propellant grains. In this test the mixture of nitrocellulose particles and triacetin was stirred with an agitator rotating at 1200 rpm. while maintaining temperature of the stirred mixture at 20 (3., taking samples at 15 minute intervals for viscosity determination at 20 C. and plotting the data on semi-log graph paper. Densified nitrocellulose particles which showed a viscosity of less than 4,000 centipoises after one hour of mixing time in this test were considered to have a satisfactory pot life.

There have been recent developments in the slurry casting art, however, which employ elevated mixing and casting temperatures in the range from about F. to about F., and it has been found that the above-described prior test procedure at 20 C. does not reliably predict satisfactory pot life at these elevated temperatures. However, when the above-described test procedure is carried out at 40 C. instead of 20 C., it does reliably predict pot life at elevated temperatures from about 90 F. to about 140 F. Densified nitrocellulose particles which show a viscosity of less than 12,000 centipoises after two hours of mixing time in the test at 40 C. have satisfactory pot life performance at the elevated temperatures from about 90 F. to about 140 F. Although the particles of substantially all lots of densified nitrocellulose prepared in accordance with application Ser. No. 358,366 have consistently showed a viscosity of less than 4,000 centipoises after one hour of mixing time at 20 C., these same lots have not consistently showed a viscosity of less than 12,000 centipoises after two hours of mixing time at 40 C.; some lots do and some do not meet the requirements of this more severe test. Theose which do not have been shown to have unsatisfactory pot life at elevated temperatures from about 90 F. to about 140 F.

It is, therefore, the primary object of this invention to provide a process for manufacturing tiny particles of densified nitrocellulose which consistently have a viscosity of less than 12,000 centipoises after two hours of mixing time at 40 C. for use in casting large propellant grains by the slurry casting process.

It is another object of this invention to provide a process for manufacturing tiny particles of densified nitrocellulose having a desirable combination of shape, size and size distribution, surface and internal structure, absolute density, bulk density, and a viscosity of less than 12,000 centipoises after two hours of mixing time at 40 C. for use in casting large propellant grains by the slurry casting process.

A further object of this invention is to provide a process for manufacturing tiny particles of densified nitrocellulose of the above-noted desirable characteristics, which particles are wet with hydrocarbon liquid instead of with water or with alcohol.

A still further object of this invention is to provide hydrocarbon-wet nitrocellulose particles of improved form for use in slurry casting of large propellant grains and in other applications where tiny particles of densified nitrocellulose are necessary or desirable.

I have now discovered that the above objects and others are accomplished in accordance with the present invention which, generally described, comprises forming a slurry with agitation of nitrocellulose fibers which are substantially free of water and also substantially free of fiber aggregates in a volatile organic liquid mixture of hydrocarbon diluent and nitrocellulose solvent in the absence of a water-soluble protective colloid, said organic liquid mixture being only a softening and swelling agent for said nitrocellulose fibers incapable of dissolving said fibers, subjecting the resulting slurry of nitrocellulose fibers to high shear agitation, whereby said fibers become softened and swollen and break down into tiny, smooth-surfaced, rounded particles, initiating hardening of the resulting comminuted and shaped particles of softened and swollen nitrocellulose by diluting the slurry with hydrocarbon diluent While continuing high shear agitation, and removing substantially all of the nitrocellulose solvent from the slurry by distillation in the presence of excess hydrocarbon diluent to complete hardening and densification of the nitrocellulose particles.

The above process produces a slurry of hard, tiny, rounded and smooth-surfaced particles of densified nitro- :ellulose of substantially irregular shapes in hydrocarbon diluent, from which the densified nitrocellulose product is readily recovered in a hydrocarbon-wet state by any convenient means, such as by gravity drainage, centrifugation, suction filtration, or the like. These densified particles have a diversity of particle sizes in the range from about microns to about 150 microns, with a substan tial majority of the particles having sizes in the range from about 30 microns to about 75 microns. The absolute density of these tiny particles is at least about 1.5 gms./cc., and normally is in the range from about 1.580 gms./ cc. to about 1.620 gms./cc., the absolute density of completely densified nitrocellulose being approximately 1.65 gms./ cc. The bulk density of these particles is at least about 45 pounds per cubic foot, and normally is in the range from about 55 to about 62 pounds per cubic foot. When magnified, these particles are seen to have smooth, dense, glazed surfaces, and generally, to have only a small degree of porosity in the interior of the particles. A composition of 30 parts by weight of densified particles of this invention uniformly mixed with 70 parts by weight of triacetin, which is a solvent-type plasticizer for nitrocellulose, forms a smooth, pourable slurry, which, after two hours of mixing time at 40 C., is still pourable and has a viscosity of less than 12,000 centipoises. This demonstrates an eminently satisfactory pot life for employment of the densified particles of this invention in the slurry casting of large propellant grains at mixing and casting temperatures in the range from about 90 F. to about 140 F. This good pot life combined with the other described properties with respect to particle shape, size and size distribution, surface and interior structure, absolute density, and bulking density characteristics makes the tiny densified particles of this invention especially well suited for use in slurry casting of large propellant grains, including slurry casting processes which employ elevated mixing and casting temperatures.

It is indeed surprising and unexpected that the process of this invention, which employs nitrocellulose fibers substantially free of water and in the absence of a watersoluble protective colloid, produces tiny densified particles of nitrocellulose with the combination of desirable shape, size and size distribution, surface and interior structure, absolute density, bulking density, and pot life which make these particles useful in the slurry casting of large propellant grains. Heretofore, the densification processes designed to produce nitrocellulose particles for use in slurry casting of large propellant grains have employed water in the densifying and shaping vehicle, and in systems containing water it has been found necessary to employ a water-soluble protective colloid as a necessary component of the densifying and shaping system. It has been postulated that the protective colloid functions in these prior systems by forming a thin protective coating or layer around each softened and swollen comminuted fragment of nitrocellulose to keep the fragments from sticking together and agglomerating into substantially larger aggregates. The protective colloid is also believed to materially aid in the rounding and shaping of the comminuted softened and swollen fragments.

At the present time it is not understood why a system substantially free of water, and in the absence of a protective colloid, in accordance with the present invention, produces densified particles having the above-enumerated desirable properties for use in slurry casting of large propellant grains. However, it is believed that all features of the invention are significant and each cooperatively contributes to the desired end result.

A significant and unexpected advantage of the present invention over previous methods which employ water in the densifying system is that certain organic lead salts such as lead salicylate, for example, can be added to the nitrocellulose during the process of densification and shaping of the nitrocellulose particles and remain substantially unaltered. This is a highly desirable advantage. In previous processes Which employ water in the densifying system, there is a pronounced tendency for organic lead salts to be chemically altered to basic lead salts. It is well known in the art of manufacturing large propellant grains, of course, that the ballistic performance of propellant grains can be regulated and controlled by the incorporation into the grain of lead compounds. Although the lead compounds can be added during the mixing of the casting slurry, a much more uniform distribution of the lead compound can be effected when the same is added to the nitrocellulose during the densification and shaping of the nitrocellulose particles, which is highly desirable in obtaining optimum uniformity of ballistic performance.

In one embodiment for practice of this invention, water-wet fibrous nitrocellulose, after conventional treatments for stabilization and viscosity adjustment, is slurried with water and beaten in a Jordan engine or similar fiber beating device to break up fiber aggregates and shorten the fibers and produce an aqueous slurry of nitrocellulose fibers substantially free of fiber aggregates. Excess water is then drained off, and the resulting Water-moist fibers are dried by passing warm air at about 50 C. through the moist fiber mass until the fibers are substantially free of water. The resulting dry nitrocellulose fibers are then introduced and dispersed at substantially room temperature with vigorous agitation, such as is produced by a Cowles Dissolver (Morehouse-Cowles, Inc., 1150' San Fernando Road, Los Angeles, Calif.) rotating at about 3,000-4,000 r.p.m., or the like, into a volatile organic liquid mixture of hydrocarbon diluent and nitrocellulose solvent, which mixture is only a softening and swelling agent for the nitrocellulose incapable of dissolving the fibers, to form a stirrable slurry of nitrocellulose fibers substantially free of water and also substantially free of fiber aggregates, in the absence of a water-soluble protective colloid. The dry nitrocellulose fibers are introduced into the organic liquid mixture as rapidly as the agitating device can disperse the fibers, either in small increments or continuously, as desired. The time required to accomplish addition and dispersal of the dried nitrocellulose fibers in the organic liquid mixture is usually quite short, amounting at most to only a few minutes. Softening and swelling of the nitrocellulose fibers commences as soon as the fibers have been introduced into the organic liquid mixture, accompanied by some breakdown of the fibers.

Alternatively, the dry nitrocellulose fibers may be initially introduced and dispersed in part or all of the hydrocarbon diluent component of the volatile organic liquid mixture of this invention with vigorous agitation to form a slurry of dry nitrocellulose fibers substantially free of fiber aggregates, and then the nitrocellulose solvent component of the volatile organic liquid mixture, together with any remaining portion of the hydrocarbon diluent component, is added to the vigorously agitated slurry.

In another embodiment for practicing this invention, water-moist nitrocellulose fibers, after excess water has been drained off of fibrous nitrocellulose which has been given conventional stabilization and viscosity treatments, and has been subjected to jordaning or similar fiber beating treatment in aqueous slurry form to break up fiber aggregates and shorten the fibers, may then be slurried with agitation in an excess of a hydrocarbon diluent which is the same as the hydrocarbon diluent to be employed as a component of the volatile organic liquid mixture of this invention. This slurry of water-moist nitrocellulose fibers in hydrocarbon diluent is then subjected to distillation until substantially all water has been removed from the nitrocellulose. Excess hydrocarbon diluent is then drained off, leaving nitrocellulose fibers substantially free of water and moist with sorbed hydrocarbon diluent, the amount of which hydrocarbon is determined by drying a sample of the moist fibers. The hydrocarbon-moist fibers are then introduced and dispersed at substantially room temperature with vigorous agitation into the volatile organic liquid mixture of hydrocarbon diluent and nitrocellulose solvent, taking into consideration that the sorbed hydrocarbon diluent associated with the nitrocellulose fibers constitutes part of the hydrocarbon diluent component of the volatile organic liquid mixture of this invention.

When all of the substantially water-free nitrocellulose fibers have become uniformly dispersed in the volatile organic liquid mixture to form a slurry of the fibers substantially free of fiber aggregates, the resulting slurry is then subjected to high shear agitation with a Kady Dispersion Mill (Kady Dispersion Corporation, 95 Botsford Place, Buffalo 16, N.Y.), or equivalent high shear agitating device, rotating at a speed of about 8,400 circumferential feet per minute in a jacketed vessel with cooling water circulating through the jacket to remove at least some of the heat generated in the slurry by the high shear agitation.

This high shear agitation is important and necessary for it definitely contributes to, and facilitates, softening and swelling of the nitrocellulose fibers, and comrninution of the softened and swollen fibers into tiny fragments in the desired size range, a majority of which are relatively uniform in size. High shear agitation also is an important feature in shaping the comminuted fragments into smoothsurfaced, rounded particles. High shear agitation is continued until substantially all of the fibers have been cornminuted and shaped, and observation of samples taken at intervals, aided by magnification, indicates no further comrninution and shaping taking place, and that a substantial equilibrium exists in the slurry with respect to size and shape of the comminuted particles. Depending somewhat on the ease of fragmentation, this may require anywhere from a few minutes to one-half hour or more.

It has been noted hereinabove that a protective colloid is not employed in the densification, sizing and shaping system of this invention. In view of prior art experience with other densification sizing and shaping processes, therefore, it would have been expected that in the absence of the protective colloid the softened and swollen nitrocellulose fibers and fiber fragments would stick together and conglomerate into undesirably large aggregates and lumps. Surprisingly, the tendency for the softened and swollen particles of nitrocellulose to stick together and conglomerate in the densifying system of this invention during the high speed, high shear agitation is negligible. It is believed that if such conglomerates are temporarily formed that they are promptly disintegrated again by the high speed, high shear agitation employed in this invention.

When observation indicates a substantial equilibrium condition to exist with respect to size and shape of the comminuted particles, the slurry is diluted with hydrocarbon diluent while continuing high shear agitation to initiate hardening of the comminuted and shaped particles. High shear agitation should preferably be continued for several minutes after dilution of the slurry with hydrocarbon diluent, permitting solvent and hydrocarbon vapors to escape from the slurry to promote further initial hardening of the particles. Dilution of the slurry with hydrocarbon diluent in this initial hardening step initiates a gradual and progressive shrinkage and densification of the particles and greatly diminishes the tendency for the particles to stick together.

After initial hardening of the comminuted particles has been accomplished by the dilution of the slurry with hydrocarbon diluent, the nitrocellulose solvent is substantially all removed by distillation in the presence of excess hydrocarbon diluent to complete the hardening and densification of the particles. The slurry desirably should be agitated during the distillation step, but such agitation need not be high shear agitation; any agitating means will suifice, for the initially hardened particles have a negligible tendency to stick together. Since a portion of the hydrocarbon diluent also distills off with the nitrocellulose solvent, sutficient hydrocarbon diluent should be added to the slurry either prior to or during distillation to insure that the particles do not agglomerate. Following removal of the nitrocellulose solvent, the slurry of hardened and densified particles of nitrocellulose in hydrocarbon diluent may, if desired, be subjected to high shear agitation for a brief period of time to disintegrate any loose aggregates formed during the distillation step. The hardened and densified particles of nitrocellulose are readily recovered in a hydrocarbon-wet state by draining off excess hydrocarbon liquid by any convenient means, such as by gravity drainage, suction filtration, centrifugation, or the like.

If desired, various additives such as nitrocellulose stabilizers, carbon black, organic lead salts, and other desirable additives which are soluble or dispersible in the organic liquid mixture can be introduced into the slurry and become very uniformly distributed into the nitrocellulose product. Insoluble ingredients desirably should be added to the slurry during the high shear comrninution operation prior to dilution of the slurry with hydrocarbon diluent to insure that they are Worked into the particles.

While production of tiny particles of densified nitrocellulose especially suited for use in casting large prOpellant grains by the slurry casting process is among the objects of this invention, it is emphasized that the utility of the densified particles of this invention is by no means limited to use in the slurry casting of large propellant grains, for these densified particles can be used in the hydrocarbon-wet state or dried in any application where commercial nitrocellulose is now used, such as lacquers, plastics, paints, adhesives, coatings, inks, impregnations, smokeless powder, and the like. In view of the tiny particle and shape, combined with high density, the densified nitrocellulose particles of this invention are also especially well suited for formulating nitrocellulose wood fillers substantially free of a tendency to shrink when overcoated with nitrocellulose lacquers or other coating materials having solvent vehicles which are solvents for nitrocellulose.

The tiny densified nitrocellulose particles of this invention are wetted with between about 20% and about 30% by weight of sorbed hydrocarbon liquid and are further characterized by being free flowing like dry sand and are relatively incompressible. For shipping purposes the ICC. requires that the densified nitrocellulose be wetted with at least 20% by Weight of liquid. The chemical characteristics of the product of this invention are substantially the same as conventional nitrocellulose, since no chemical action is involved in the process of producing this product.

The general nature of the invention has been set forth and the following examples are presented as specific illustrations thereof. All parts and percentages are by weight unless otherwise stated.

Example 1 Water-wet fibrous nitrocellulose, 12.6% nitrogen by weight, 40 seconds -i11ch falling ball viscosity measured in accordance with Military Spec. JAN-N-244, after conventional treatments for stabilization and viscosity adjustment, was slurried with water and jordaned to break up fiber aggregates and shorten the fibers and produce a slurry of nitrocellulose fibers substantially free of fiber aggregates. Excess water was then drained off, and the resulting water-moist fibers were dried by passing heated air at about 50 C. through the moist fiber mass until the fibers were substantially free of water, i.e., to constant weight.

Three hundred fifty (350) parts of the resulting dry nitrocellulose fibers were then introduced and dispersed at room temperature with agitation in an organic liquid mixture of 806 parts of methyl ethyl ketone and 731 parts of heptane having dissolved therein 6 parts of 2- nitrodiphenylamine in a stainless steel vessel equipped with a Cowles Dissolver to form a stirrable slurry of the dried nitrocellulose fibers substantially free of fiber aggregates. This required about 5 minutes of agitation with the Cowles Dissolver operating at approximately 4,000 rpm. The ratio of methyl ethyl ketone to nitrocellulose was 2.30, and the ratio of methyl ethyl ketone to heptane was 1.10 by weight.

The charge was then transferred to a jacketed Kady Dispersion Mill provided with a cover, with cooling water circulating through the jacket, and was subjected to high shear agitation at approximately 8,400 circumferential feet per minute in the covered Kady Dispersion Mill for approximately 25 minutes. The temperature in the slurry reached, and was maintained by jacket cooling, in the range between about 60 C. and 64 C. during this period of high shear agitation, and the slurry progressively thickened. Approximately 68.4 parts of heptane were added to the slurry during this period of high shear agitation in the covered Kady Dispersion Mill to counteract the progressive thickening of the slurry. After 25 minutes of high shear agitation in the covered Kady Mill, the slurry was diluted with an additional 137 parts of heptane, the cover of the Kady Mill was removed, and the high shear agitation was continued for approximately minutes. During this latter period of high shear agitation there was appreciable evaporation of the methyl ethyl ketone and heptane components from the slurry, and additional heptane was added as needed to maintain the liquid level in the mill.

The charge was then diluted with approximately 350 parts of additional heptane and transferred to a distillation kettle provided with a conventional propeller type agitator, and the remaining methyl ethyl ketone was substantially all removed, adding heptane as needed to maintain the liquid level in the distillation vessel. Distillation was discontinued when the distillation temperature reached approximately 91 C. The charge was again transferred to the Kady Mill where it was again subjected to high speed, high shear agitation for approximately one minute to disintegrate a few loosely agglomerated aggregates formed during the distillation, after which the slurry of tiny, densified nitrocellulose particles in heptane was deliquidified on a suction filter to 70% total solids by weight.

The resulting densified nitrocellulose particles had an absolute density, dry, of 1.609 gins/cc. and a bulk density, dry basis, of 55.7 lbs/cu. ft. The overall particle size was in the range from microns to 120 microns with a majority of the particles in the range from 40 microns to 60 microns. Visual examination with the aid of magnification showed the particles to be smooth and rounded with dense glazed surfaces, and for the most part to be of irregular shapes. The dried material flowed easily like dry sand.

Thirty percent by weight of the dried particles were mixed with 70% by weight of triacctin to produce a smooth, pourable slurry which was then stirred by agitation at 1,200 r.p.m. for 2 hours at C. The viscosity build-up of this slurry with time at 40 C. follows:

g Viscosity (centipoises): Time (minutes) 15 110 30 155 45 215 60 285 75 465 .t 635 1090 The slurry was readily pourable after 2 hours of mixing time at 40 (3., thus demonstrating that the densified particles were satisfactory and well suited for use in slurry casting of large propellant grains, including mixing and casting at elevated temperatures.

The -inch falling ball viscosity characteristic of the nitrocellulose was measured in accordance with Military Spec. JAN-N-244 on a 10% by weight solution of the nitrocellulose in a solvent composed of 11.1% denatured ethyl alcohol and 88.9% acetone by weight at 25 C., noting the time in seconds for a -inch steel ball to fall freely 10 inches through the solution.

Example 2 The same water-wet fibrous nitrocellulose as employed in Example 1, after conventional stabilization and viscosity adjustment treatments, was jordaned and dried as described in Example 1. The resulting dry nitrocellulose fibers were then wetted with an equal weight of heptane.

Seven hundred (700) parts of the resulting heptanewet nitrocellulose fibers, 350 parts, dry weight, were then introduced and dispersed with agitation at room temperature by means of a Cowles Dissolver rotating at approximately 4,000 r.p.m. in an organic liquid mixture of 806 parts of methyl ethyl ketone and 381 parts of heptane having dissolved therein 6 parts of 2-nitrodiphenylamine in a stainless steel vessel to produce a stirrable slurry of nitrocellulose fibers substantially free of Water and also substantially free of fiber aggregates. The ratio of methyl ethyl ketone to nitrocellulose was 2.30, and the ratio of methyl ethyl ketone to heptane was 1.10 by weight.

After 5 minutes of Cowles agitation, the charge was transferred to a jacketed Kady Dispersion Mill provided with a cover, with cooling water circulating through the jacket, and was subjected to high shear agitation at approximately 8,400 circumferential feet per minute in the covered Kady Dispersion Mill for approximately 25 minutes. The temperature in the slurry reached, and was maintained by jacket cooling, in the range between about 60 C. and 64 C. during this period of high shear agitation, and the slurry progressively thickened. Approximately 34.2 parts of heptane were added to the slurry during this period of high shear agitation to counteract the progressive thickening of the slurry. After 25 minutes of high shear agitation in the covered Kady Mill, the slurry was diluted with an additional 137 parts of heptane, the cover of the Kady Mill was removed, and the high shear agitation was continued for approximately 10 minutes. During this latter period of high shear agitation there was appreciable evaporation of the methyl ethyl ketone and heptane components of the slurry, and additional heptane was added as needed to maintain the liquid level in the mill.

The charge was then diluted with approximately 350 parts of additional heptane and transferred to a distillation kettle provided with a conventional propeller type agitator, and the remaining methyl ethyl ketone was substantially all removed, adding heptane as needed to maintain the liquid level in the distillation vessel. Distillation was discontinued when the distillation temperature reached approximately 91 C. The charge was again transferred to the Kady Mill where it was again subjected to high speed, high shear agitation for approximately 1 minute to disintegrate a few loosely agglomerated aggre- 9 gates formed during the distillation, after which the slurry of tiny, densified nitrocellulose particles in heptane was deliquified on a suction filter to 70% total solids by weight.

The resulting densified nitrocellulose particles had an absolute density, dry, of 1.571 gms./cc., and a bulk density, dry, of 61.2 lbs/cu. ft. The overall particle size was in the range from to 125 microns with a majority of the particles in the range from to 50 microns. Visual examination with the aid of magnification showed the particles to be smooth and rounded with dense glazed surfaces, and for the most part to be of irregular shapes.

Thirty percent by weight of the dried particles were mixed with 70% by weight of triacetin to produce a smooth, pourable slurry which was then stirred by agitation at 1,200 rpm. for 2 hours at C. The viscosity build-up of this slurry with time at 40 C. follows:

Viscosity (centipoises) Time (minutes) The slurry was readily pourable after 2 hours of mixing time at 40 0., thus demonstrating that the densified particles were satisfactory and well suited for use in slurry casting of large propellant grains, including mixing and casting at elevated temperatures.

The tiny densified nitrocellulose particles of this invention can be produced from any fibrous nitrocellulose, obtained by nitrating natural or artificial cellulose fibers, such as cotton, purified cotton linters, purified wood pulp, regenerated cellulose fibers, and the like, in such forms as picked linters, shredded wood pulp, fluifed bulk linters, finely ground or cut fibers, fiber aggregate particles, and the like. However, it is important that the fibrous nitrocellulose be substantially free of fiber aggregates for use in this invention, for such aggregates interfere with proper comminution and lead to formation of undesirably large particles.

Accordingly, it is both desirable and preferable to initially subject the fibrous nitrocellulose to a conventional jordaning, or similar fiber beating treatment, to break up fiber aggregates and generally shorten the fibers prior to use in this invention. To accomplish this, water-wet fibrous nitrocellulose, after conventional treatments for stabilization and viscosity adjustments, is slurried with water to the consistency of a conventional pulping slurry and is beaten in a Jordan engine, or similar fiber beating device, to break up and disintegrate fiber aggregates and generally shorten the fibers.

This invention requires nitrocellulose fibers which are also substantially free of water, and it has been pointed out hereinbefore that the fibers, after draining 01f excess Water, may be dried by passing warm air through the fiber mass, or by distilling off the residual water in the presence of a large excess of hydrocarbon diluent. The water may also be removed by conventional and wellknown alcohol dehydration methods.

The term substantially free of water does not necessarily mean that every last trace of water has been removed to produce a truly anhydrous product. The nitrocellulose fibers substantially free of water can and usually do contain a small residuum of sorbed water, usually substantially less than 1% by weight of the dried fibers. This residuum of sorbed water, however, is without significant interference in the process of this invention.

Substantially all commercial types and grades of fibrous nitrocellulose are suitable for the purposes of this invention, having nitrogen contents from about 10.9% to about 13.5% nitrogen by weight, and of any viscosity charac- 10 teristic from the very low viscosity 10 centipoise type to exceedingly high viscosity types as exemplified by dynamite grade nitrocellulose.

The substantially water-free nitrocellulose fibers are introduced and dispersed in the organic liquid softening and swelling medium of this invention with vigorous agitation to form a uniformly smooth, readily stirrable slurry substantially free of fiber aggregates, and the upper practical limit for the amount of nitrocellulose fibers in the slurry is governed by the ability to agitate the slurry effectively by high speed, high shear agitation. Generally, slur-ries containing from about 16.5% to about 18.5% by weight of nitrocellulose, dry weight, have been employed and optimum results were obtained with slurries containing about 18.5% by weight of nitrocellulose. Slurries containing more than about 18.5% of nitrocellulose are usually of too high a consistency to be effectively agitated by the high speed, high shear device.

As noted hereinbefore, the volatile organic liquid mixture which is employed as the medium for the comminution, shaping, and densification of the nitrocellulose is a mixture of hydrocarbon diluent and nitrocellulose solvent. It is important and necessary for the hydrocarbon diluent and nitrocellulose solvent to be proportioned in the mixture so that the mixture is only a swelling and softening agent for nitrocellulose fibers, incapable of dissolving the fibers. It will be apparent, of course, that suitable proportions of hydrocarbon diluent and nitrocellulose solvent to obtain this objective will vary depending principally on the particular hydrocarbon diluent and nitrocellulose solvent selected, and to a minor extent on the nitrogen content and viscosity characteristic of the nitrocellulose to be comminuted and densified. However, with any particular selection of hydrocarbon diluent and nitrocellulose solvent, it is a simple matter to carry out a preliminary trial by slurrying the desired nitrocellulose in the selected hydrocarbon diluent and then progressively adding the selected nitrocellulose solvent with agitation until the point is reached where the mixture begins to swell the fibers. It is then only necessary in practicing this invention to make minor increases or decreases in the ratio of hydrocarbon diluent to nitrocellulose solvent to obtain the desired shapes, density, bulk density, range of particle sizes, etc. When employing mixtures containing heptane and methyl ethyl ketone as the medium for comminution, shaping, and densification of nitrocellulose of 12.6% nitrogen and 40 seconds -inch falling ball viscosity, the optimum ratio of methyl ethyl ketone to heptane by weight is about 1.10, and the ratio of methyl ethyl ketone to nitrocellulose by weight is about 2.30.

Any volatile hydrocarbon which is liquid at ordinary temperatures and atmospheric pressure may be employed for the purposes of this invention including aliphatic, cycloaliphatic, aromatic, arylaliphatic and aliphaticaryl hydrocarbons, and mixtures of any of these. Some typical hydrocarbons include, by way of example, hexane, heptane, octane, isooctane, nonane, and the like, various proprietary petroleum distillate cuts such as textile spirits, mineral spirits, lactol spirits, VM & P naphtha, gasoline kerosene, and the like, cyclepentane, cyclohexane, methyl cyclohexane, benzene, toluene, xylene, ethyl benzene, styrene, a-methyl styrene, various proprietary aromatic hydrocarbon distillate cuts, mixtures of aliphatic and aromatic hydrocarbons, and the like. Aliphatic hydrocarbons which boil in the range of heptane or higher are preferred and heptane is especially preferred.

Nitrocellulose solvents suitable for the purposes of this invention are the lower molecular weight ketones, esters glycol ether-alcohols and glycol ether-esters. Some typica nitrocellulose solvents which are suitable for practice 0: this invention include, for example, methyl formate, ethy formate, methyl acetate, ethyl acetate, isopropyl acetate propyl acetate, butyl acetate, methyl propionate, ethy propionate, acetone, methyl ethyl ketone, diethyl ketone methyl propyl ketone, methyl isobutyl ketone, ethylene glycol monomethyl ether, ethylene glycol dimethyl ether, ethylene glycol monoethyl ether, ethylene glycol diethyl ether, methoxyethyl acetate, ethylene glycol diacetate, and the like. Preferably, the nitrocellulose solvent component of the organic liquid mixture of this invention should have a boiling point below the boiling point of the hydrocarbon diluent component, or should form a minimum boiling mixture therewith. *Methyl ethyl ketone is especially preferred for the purposes of this invention.

What I claim and desire to protect by Letters Patent is:

1. A process for preparing a densified, fine-particle nitrocellulose product which comprises (a) forming a slurry with agitation of nitrocellulose fibers which are substantially free of water and also substantially free of fiber aggregates in a volatile organic liquid mixture of hydrocarbon diluent and nitrocellulose solvent in the absence of a water-soluble protective colloid;

(b) said organic liquid mixture being only a softening and swelling agent for said nitrocellulose fibers incapable of dissolving said fibers;

(c) subjecting the resulting slurry of nitrocellulose fibers to high shear agitation, whereby said fibers become softened and swollen and break down into tiny, smooth-surfaced, rounded particles;

(cl) initiating hardening of the resulting comminuted and shaped particles of softened and swollen nitrocellulose by diluting the slurry with hydrocarbon diluent while continuing high shear agitation;

(e) and removing substantially all of the nitrocellulose solvent from the slurry by distillation in the presence of excess hydrocarbon diluent to complete hardening and densification of the nitrocellulose particles.

2. A process in accordance with claim 1 in which the nitrocellulose fibers substantially free of water are initially dispersed with agitation in at least part of the hydrocarbon diluent component of said volatile organic liquid mixture, and the nitrocellulose solvent component of said volatile organic liquid mixture, together with any remaining portion of the hydrocarbon diluent component, is added to the vigorously agitated slurry.

3. A process in accordance with claim 1 in which water- Wet nitrocellulose fibers are initially subjected to distillation in the presence of an excess of hydrocarbon diluent, which hydrocarbon diluent is the same as the hydrocarbon diluent component of said volatile organic liquid mixture, until substantially all water is removed from the nitrocellulose, draining off excess hydrocarbon diluent, and introducing and dispersing the resulting hydrocarbon diluent moist nitrocellulose fibers with agitation into a mixture of the nitrocellulose solvent component and the remaining portion of the hydrocarbon diluent component of said volatile organic liquid mixture.

4. A process in accordance with claim 1 in which the hydrocarbon diluent is heptane and the nitrocellulose solvent is methyl ethyl ketone.

5. A process in accordance with claim 4 in which the nitrocellulose has a nitrogen content of at least about 12.6% by weight, the initial ratio of methyl ethyl ketone to heptane in the slurry is about 1.10 by weight, and the ratio of methyl ethyl ketone to nitrocellulose is about 2.30 by weight.

6. A process in accordance with claim 1 in which the slurry of hardened and densified nitrocellulose particles in hydrocarbon diluent after distillation is again subjected to high shear agitation to disintegrate any loose aggregates formed during the distillation step.

References Cited UNITED STATES PATENTS 2,715,574 8/1955 Cox l492 2,885,736 5/1959 ONeill l492 X 2,946,673 7/1960 Grassie l492 X 3,236,702 2/ 1966 Sapiego l492 3,284,253 11/1966 Enders et al. l492 BENJAMIN R. PADGETT, Primary Examiner. 

1. A PROCESS FOR PREPARING A DENSIFIED, FINE-PARTICLE NITROCELLULOSE PRODUCT WHICH COMPRISES (A) FORMING A SLURRY WITH AGITATION OF NITROCELLULOSE FIBERS WHICH ARE SUBSTANTIALLY FREE OF WATER AND ALSO SUBSTANTIALLY FREE OF FIBER AGGREGATES IN A VOLATILE ORGANIC LIQUID MIXTURE OF HYDROCARBON DILUENT AND NITROCELLULOSE SOLVENT IN THE ABSENCE OF A WATER-SOLUBLE PROTECTIVE COLLOID; (B) SAID ORGANIC LIQUID MIXTURE BEING ONLY A SOFTENING AND SWELLING AGENT OFR SAID NITROCELLULOSE FIBERS INCAPABLE OF DISSOLVING SAID FIBERS; (C) SUBJECTING THE RESULTING SLURRY OF NITROCELLULOSE FIBERS TO HIGH SHEAR AGITATION, WHEREBY SAID FIBERS BECOME SOFTENED AND SWOLLEN AND BREAK DOWN INTO TINY, SMOOTH-SURFACED, ROUNDED PARTICLES; (D) INITIATING HARDENING OF THE RESULTING COMMINUTED AND SHAPED PARTICLES OF SOFTENED AND SWOLLEN NITROCELLULOSE BY DILUTING THE SLURRY WITH HYDROCARBON DILUENT WHILE CONTINUING HIGH SHEAR AGITATION; (E) AND REMOVING SUBSTNATIALLY ALL OF THE NITROCELLULOSE SOLVENT FROM THE SLURRY BY DISTILLATION IN THE PRESENCE OF EXCESS HYDRCARBON DILUENT TO COMPLETE HARDENING AND DENSIFICATION OF THE NITROCELLULSOE PARTICLES. 